1. Field of the Invention
The present invention relates generally to spacecraft attitude control systems and in particular to estimating periodic disturbance torques acting on an orbiting spacecraft.
2. Brief Description of Related Developments
Orbiting spacecraft are typically subjected to a periodic disturbance torque environment large enough to perturb the spacecraft from its desired attitude. The effects of this periodic disturbance torque environment could be mitigated if the disturbances can be accurately predicted or estimated. Solar torques represent the dominant periodic disturbance environment on an Earth-pointing satellite, with a period equal to the spacecraft solar day(e.g., twenty-four hours for a geosynchronous satellite). There is also a large component caused by the inertia mass properties of the spacecraft. These torques are generally dominated by constant body-fixed torques and signals at orbit rate and twice orbit rate.
Feedback control algorithms are highly effective at mitigating these disturbances given the right sensors. Most orbiting spacecraft employ Earth sensors, which provide continuous measurements of roll and pitch. Coupling these measurements with an appropriate set of actuators, such as momentum wheels and/or thrusters, the roll and pitch angles may be kept close to commanded roll and pitch angles using well established feedback control techniques. However, continuous measurements of yaw are not available on many spacecraft, making these systems highly susceptible to disturbances perturbing yaw. The current art uses a pitch-axis angular momentum bias to couple the roll and yaw axis dynamics, thereby improving yaw pointing through tight roll control. Further improvements in yaw pointing can be made if the disturbance torques are accurately predicted or estimated and counteracted by the actuators.
In addition to improved yaw pointing, knowledge of the disturbance torque environment can lead to better momentum management. Momentum wheels are used on many spacecraft to store accumulated momentum resulting from the disturbance torque environment. These wheels have a limited storage capacity, so many spacecraft use fuel-expending thrusters to unload momentum. Typically, the momentum unload is triggered whenever the wheel speed crosses a threshold. This is known as a momentum management strategy. If the disturbance torque profile is known in advance, the wheel speed trajectory can be predicted, which can be used for a fuel-efficient momentum management strategy.
The present invention is directed to a spacecraft embedded in a reference frame rotating relative to inertial space. In one embodiment, the spacecraft comprises actuators for maneuvering the spacecraft with respect to the reference frame, an attitude measurement device that measures the pitch and roll attitude of the spacecraft with respect to the reference frame, a control device adapted to keep the roll and pitch angles of the spacecraft close to the commanded roll and pitch angles, and a harmonic torque estimator adapted to read the commanded angular velocity of the spacecraft relative to an inertial frame, read momentum wheel speeds, read known predicted external torques and combines angular velocity, measured wheel speed and known external torque to produce an estimated observable periodic torque.
In one aspect, the present invention is directed to a spacecraft attitude control system. In one embodiment, the control system comprises a harmonic torque estimator adapted to estimate a periodic disturbance torque signal and a periodic momentum profile signal based on disturbances observable from the available spacecraft sensor. The system also includes an attitude profile generator adapted to provide reference attitude command and a rate command for the spacecraft to follow, a feedback control device adapted to measure roll and pitch angles to the commanded roll and pitch angles issued by the attitude profile generator and output a torque that is combined with the periodic disturbance torque estimate to form a momentum wheel torque command, and a momentum management device adapted to use the estimated periodic momentum profile to improve momentum management of the spacecraft.
In another aspect, the present invention is directed to a method of controlling a spacecraft. In one embodiment, the method includes measuring a pitch and roll attitude of the spacecraft, commanding the spacecraft actuators based on a pitch and roll attitude measurements in order to keep the roll and pitch angles close to desired roll and pitch angles, estimating a periodic observable torque, and feeding forward the estimated periodic torque to the spacecraft actuators in order to improve yaw pointing.